Laser fabrication of continuous nanofibers

ABSTRACT

This invention provides a continuous process of making continuous nanofibers of all kinds, such as SiC, BN, AlN, and C. Laser heating a vapor of feed-material made of all atomic elements needed to grow chosen nanofibers results in growth of nanofibers onto seed-nanostructures attached to a filament, which is then pulled up continuously at a rate controlled by a rate of growth of the nanofibers. More feed-material is supplied at a rate sufficient to enable the nanofibers to grow longer continuously without limit. Laser light focused into a doughnut shape provides a photon density gradient, which constrains the nanofibers to grow parallel to each other and in the form of cylinders, so that industrially useful structures like cables and cylinders can be made in one low cost operation and in large quantities.

BACKGROUND OF THE INVENTION

This invention relates to a continuous-flow process of makingcontinuous-length nanofibers by laser vaporization.

The fantastic properties and applications of nanotubes have beendescribed in a presentation of a new field called “Fractal TubeReinforcement Microengineering” by Russell in 32^(nd) InternationalSAMPE Technical Conference, p. 224 (Nov. 5-9, 2000). There has beengreat interest in using nanotubes as building blocks to construct allkinds of products with superior performance, but nanotubes have been fartoo costly and of insufficient length and quantity to be useful for mostindustrial applications. Smalley, et al. in U.S. Pat. No. 6,183,714disclose pulse laser vaporization of carbon mixed with one or more GroupVIII transition metals to make a carbon nanotube. They then use a secondlaser pulse to maintain this nanotube end in an annealing zone, whichallows growth of ropes of nanotubes. These ropes, however, are notcontinuous-length ropes and not attached to anything that can be pulledor wound. So, this process is not continuous and must be stopped torecover nanotube ropes from condensed vapor.

Kalaugher reported in “Nanotubes go to great lengths”, Nanotechweb.org(Mar. 11, 2004) that Windle and colleagues at Cambridge University haveused chemical vapor deposition in a furnace of ethanol with ferroceneand thiophene and catalyzed with iron to make ribbons of carbonnanotubes. However, they do not disclose any way to apply tension to thenanofibers to constrain them to grow parallel to each other withcontrolled geometry to form useful structures.

So, there remains a need to make continuous-length nanofiber inparallel, controlled arrangements in order to make commercially usefulstructures economically in large quantities.

SUMMARY OF THE INVENTION

A main object of the instant invention is to provide a continuous laserfabrication process of making all kinds of nanofibers of unlimitedlength. This is accomplished herein by laser vaporization of afeed-material made of all of the elements required to grow a chosen kindof nanofiber in a reactor, such that new nanofibers of the chosen kindgrow onto seed-nanostructures attached to a filament. More feed-materialis supplied to the reactor as needed to support the continuous growth ofnew nanofibers, which are pulled out and wound up at a rate controlledby their growth rate in a continuous-flow process.

Another object is to provide a process of controlling the growth of newnanofibers in order to make useful structures like cables and cylindersin one continuous operation. This is accomplished herein by applyingtension to the growing nanofibers by focusing at least one laser lightbeam into the shape of a doughnut at a vapor of feed-material in areaction zone. This provides a photon density gradient, which gathersatoms of the vapor together and constrains the growth of new nanofibersto be parallel to each other and to assume a cylindrical form. Inanother aspect an electrical field is applied to align the newnanofibers and constrain them into chosen shapes and useful structures.

These and other objects and advantages of the invention will be betterunderstood from the following detailed description of preferredembodiments of the inventive process when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 a is a schematic cross-sectional view of a laser-assistedcontinuous nanofiber reactor.

FIG. 1 b is an enlarged view of section X in FIG. 1 a showing nanofibersattached to filament.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In a first embodiment seed-nanostructures are placed in a reactor 1shown in FIG. 1 a, which is capable of sustaining pressure. The pressurein the reactor 1 depends on the type of nanofibers to be built andcontrols their rate of growth to a manageable rate. Theseed-nanostructures comprise nanotubes of any type and chemicalcomposition including fullerenes and nanotube ropes together withcatalyst nanoparticles, such as a mixture of Co and Ni. The catalystnanoparticles are, alternatively, any of the transition elements orcombinations thereof. In another example, seed-nanostructures compriseelements from which nanofibers are built. Nanostructures are defined tobe structures less than 1 micron in thickness. An electric charge isimparted to an assembly of these seed-nanostructures relative to afilament 2 in the reactor 1 by means of a static charge generator untilthey become aligned relative to each other. Oxygen and water vapor areevacuated and inert gas is injected in the reactor 1. The preferredinert gas is a gas selected from the group consisting of argon, helium,and any combination of these. Each lower tip of at least one filament 2is heated by reflecting at least one input laser light beam 3 frommirror 8 to focusing mirror 7 and focusing the laser beam 3 on the lowertip by the focusing mirror 7 until the lower tip becomes molten hot. Apreferred filament is one made of sapphire having a tapered lower tip.Another preferred filament is a nanofiber. The filament 2 may be made ofany other material. The lower tip of the at least one filament 2 is thenmoved out of a path of the focused laser beam 3 toward theseed-nanostructures such that they become attached to the filament 2. Inone example the seed-nanostructures become attached to the filament 2 byembedding themselves into the molten tip. In another example they areadhered to the filament 2 by means of an applied adhesive. In anotherexample they become attached by means of the electrostatic charge placedon them. A preferred laser emits a light beam 3 with a power of at least10 mW in the visible frequency range. Visible light lasers are availablefrom New Lamda Corp., Clearwater, Fla. Feed-material 4 is provided inthe reactor 1, which comprises all elements needed to build newnanofibers 5 including a total of at most 5 atom % of at least onetransition element. More preferably the feed-material 4 comprises allelements needed to build new nanofibers 5 including a total of at most 3atom % of at least 3 transition elements. Two examples of highlyeffective transition element combinations in catalyzing new nanofibergrowth are 1 atom % each of Ni, Fe, & Co, and Y, Ir, & Pt. In this paperatom % is defined as the percentage of atoms relative to all other atomspresent. Most preferably only a negligible amount of transition elementsare included and only in the beginning of the inventive process, becausethis costly part of the process is minimized. In one example thefeed-material 4 is in a solid state. In a second example thefeed-material is in a liquid state. In a third example the feed-materialis in a gas state. Examples of feed-material 4 are carbon, hydrocarbons,fullerenes, SiC, BN, AlN, and any combination of these. Thefeed-material 4 is then heated to form vapor. A preferred way to heatthe feed-material 4 is by means of at least one infrared laser beam 6.Infrared lasers are available from New Lambda Corp., Clearwater, Fla. Inother examples electric arc, plasma arc, and radio frequency inductionare used as means to heat the feed-material 4. The vapor is heated bymeans of the focused laser beam 3 sufficiently so that new nanofibersgrow in a local region of the focus of the beam 3. A preferred reactiontemperature of the vapor is 500° C. to 1400° C. More preferably thereaction temperature of the vapor is 600° C. to 1250° C. The at leastone laser beam 3 creates a photon density gradient, which gathers atomsand molecules from the vapor together and constrains them to assembleinto new nanofibers 5. In this paper, nanofiber is defined as any fiberthat has a thickness of less than 1 micron. The new nanofibers 5 includesingle-wall nanotubes. In another example the new nanofibers 5 includemulti-wall nanotubes. In another example the new nanofibers 5 include amixture of single-wall and multi-wall nanotubes. The well-knowntechnology of using a photon density gradient from highly focused lightto manipulate matter is documented by Plewa et al. in “Processing CarbonNanotubes with Holographic Optical Tweezers”,Physics.nyu.edu/grierlab/nanotube3b (Feb. 14, 2004), and by Dholakia etal. in “Optical tweezers: the next generation”,Nanotechweb.org/articles/feature/1/10/2/1 (October 2002). The lower tipof the filament 2 is then raised to a position just above the focus ofthe beam 3 such that new nanofibers form from the vapor on theseed-nanostructures. The new nanofibers 5 are structurally continuouswith the seed-nanostructures. In one example the new nanofibers 5 arechemically identical with the seed-nanostructures. In another examplethe new nanofibers 5 are made of different elements than theseed-nanostructures. The filament 2 is then pulled out with the newnanofibers 5 attached as shown in FIG. 1 b at a rate controlled to matcha rate of new nanofiber growth. Tension is provided on the newnanofibers 5 as they grow by at least one tensioning means. Thisconstrains the new nanofibers 5 to grow parallel to each other.Feed-material 4 is continuously provided, as required, to allowcontinuous growth of new nanofibers 5 in a continuous-flow process. Themeans of providing tension is by focusing the at least one laser beam 3into a toroid or doughnut shape. Preferably 3 laser beams 3 are usedtogether to manipulate the nanofiber growth. This further constrains thenew nanofibers 5 to assemble in a substantially cylindrical shape, sothat cables, cylinders and hollow cylinders and other structures aremanufactured in one efficient operation. Bonding agents may be used inthis process to bond the new nanofibers 5 comprising these structures.Another means of providing tension and manipulating the new nanofibers 5to control a final product geometry is by applying an electric field tothe new nanofibers by an electric field generator. As the filament 2 ispulled out with the new nanofibers 5 attached it is wound up onto aspool, for example. The length of the new nanofibers 5 is not limited.The chemical make-up of the new nanofibers 5 depends on the type offeed-material used. Examples of new nanofibers 5 are C, SiC, BN, AlN,and any combination of these.

In a second embodiment the seed-nanostructures are replaced withseed-nanotubes, which are comprised of nanotubes. In another example theseed-nanotubes are comprised of short sections of nanotube ropes. Thereare no transition elements used in this embodiment to catalyze formationof new nanofibers 5. An electric charge is placed on the at least onefilament 2 relative to the seed-nanotubes. In all other respects theprocess steps are the same in this embodiment as in the firstembodiment.

While there is described herein certain specific process steps embodyingthe invention, it will be manifest to those skilled in the art thatmodifications may be made without departing from the spirit and thescope of the underlying inventive concept. The present invention shallnot be limited to the particular processes herein shown and describedexcept by the scope of the appended claims.

1. A process of making continuous nanofibers comprising the steps of:providing seed-nanostructures in a reactor capable of maintaining apositive internal pressure; applying a static electric charge to anassembly of said seed-nanostructures in said reactor by means of astatic electric charge generator sufficiently so that saidseed-nanostructures become aligned relative to each other; removingoxygen and water and injecting inert gas in said reactor with saidseed-nanostructures; heating each lower tip of at least one filament byfocusing at least one laser light beam on said tip by means of at leastone laser until said tip becomes molten hot; moving said tip of saidfilament out of a path of said beam toward said seed-nanostructures suchthat said seed-nanostructures become attached to said tip of saidfilament; providing feed-material comprised of all atomic elements fromwhich new nanofibers are built; making a vapor from said feed-materialby heating said feed-material; heating said vapor to a temperaturesufficient to cause new nanofiber growth and gathering atoms of saidvapor together in a local region of said laser light beam by focusingsaid beam from said laser; raising said tip of said filament to aposition just above said beam such that said new nanofibers form fromsaid vapor in a continuous structure with said seed-nanostructures;pulling out said filament with attached said new nanofibers at a ratecontrolled to match a rate of new nanofiber growth; providing tension onsaid new nanofibers as they grow by at least one tensioning means; andconstraining said new nanofibers to grow substantially parallel to eachother by said tensioning means.
 2. The process of claim 1 wherein saidprocess is a continuous-flow process such that said feed-material iscontinuously provided as required to allow continuous growth of said newnanofibers.
 3. The process of claim 2 wherein the step of providing saidfeed-material further comprises including in said feed-material a totalof at most 5 atom % of at least one transition element selected from thegroup consisting of all transition elements.
 4. The process of claim 3wherein the step of providing tension further comprises converging saidlaser light beam such that a photon density gradient is provided havinga toroidal shape, said laser being a visible light laser, and the stepof constraining said new nanofibers to grow substantially parallel toeach other further comprises constraining said new nanofibers toassemble into a cylindrical shape by means of said photon densitygradient.
 5. The process of claim 4 wherein the step of providingtension further comprises applying an electrical field to said newnanofibers by means of an electric field generator.
 6. The process ofclaim 4 wherein the step of providing feed-material further comprisesproviding carbon feed-material, including in said feed-material a totalof at most 3 atom % of at least three transition elements selected fromthe group consisting of all transition elements, and said new nanofibersare carbon nanofibers.
 7. The process of claim 5 wherein the step ofproviding feed-material further comprises providing carbonfeed-material, including in said feed-material a total of at most 3 atom% of at least three transition elements selected from the groupconsisting of all transition elements, and said new nanofibers arecarbon nanofibers.
 8. The process of claim 6 wherein the step of pullingup said filament further comprises winding said filament together withattached said new nanofibers, and the step of heating said feed-materialfurther comprises heating said feed-material by means of an infraredlaser.
 9. The process of claim 7 wherein the step of pulling out saidfilament further comprises winding said filament together with attachedsaid new nanofibers, and the step of heating said feed-material furthercomprises heating said feed-material by means of at least one infraredlaser.
 10. The process of claim 5 wherein the step of providingfeed-material further comprises providing boron nitride in saidfeed-material.
 11. The process of claim 5 wherein the step of providingfeed-material further comprises providing silicon carbide in saidfeed-material.
 12. The process of claim 5 wherein the step of providingfeed-material further comprises providing aluminum nitride in saidfeed-material.
 13. A process of making continuous nanofibers comprisingthe steps of: providing seed-nanotubes in a reactor; applying a staticcharge to at least one filament in said reactor by means of a staticcharge generator sufficiently so that said seed-nanotubes become alignedrelative to each other; removing oxygen and water and injecting inertgas in said reactor; heating each lower tip of said at least onefilament by focusing at least one laser light beam on said tip by meansof at least one laser until said tip becomes molten hot; moving said tipof said filament out of a path of said beam toward said seed-nanotubessuch that said seed-nanotubes become attached to said tip of saidfilament; providing feed-material including all elements from which newnanofibers are built; making a vapor from said feed-material by heatingsaid feed-material by at least one heating means; heating said vapor toa temperature sufficient to cause new nanofiber growth and gatheringatoms of said vapor together in a local region of said laser light beamby focusing said beam from said laser; raising said tip of said filamentto a position just above said beam such that new nanofibers form fromsaid vapor in a continuous structure with said seed-nanotubes; pullingout said filament with attached said new nanofibers at a rate controlledto match a rate of new nanofiber growth; providing tension on said newnanofibers as they grow by converging said laser light beam such that aphoton density gradient is provided having a toroidal shape; andconstraining said new nanofibers to grow substantially parallel to eachother in a substantially cylindrical shape by means of said photondensity gradient.
 14. The process of claim 13 wherein said process is acontinuous-flow process such that said feed-material is continuouslyprovided as required to allow continuous growth of said new nanofibers.15. The process of claim 14 wherein the step of providing tensionfurther comprises applying an electric field to said new nanofibers bymeans of an electric field generator.
 16. The process of claim 15wherein the step of pulling out said filament further comprises windingsaid filament together with attached said new nanofibers, and the stepof heating said feed-material further comprises heating saidfeed-material by means of an infrared laser.
 17. The process of claim 13wherein the step of providing feed-material further comprises providingfeed-material that comprises a carbon material selected from the groupconsisting of carbon, and silicon carbide, and any combination of these.18. The process of claim 16 wherein the step of providing feed-materialfurther comprises providing feed-material that comprises a carbonmaterial selected from the group consisting of carbon, silicon carbide,and any combination of these.
 19. The process of claim 13 wherein thestep of providing feed-material further comprises providingfeed-material that comprises a nitride selected from the groupconsisting of boron nitride, and aluminum nitride, and any combinationof these.
 20. The process of claim 16 wherein the step of providingfeed-material further comprises providing feed-material that comprises anitride selected from the group consisting of boron nitride, andaluminum nitride, and any combination of these.